1. Field of the Invention
The present invention relates to a transflective liquid crystal display device, and more particularly, a transflective liquid crystal display device capable of implementing an entire pixel region both in a reflection mode and in a transmission mode by reflecting external light (ray) incident on a lower side of a liquid crystal display panel and transmitting light incident from a backlight unit.
2. Discussion of Background Art
In general, a liquid crystal display (LCD) device is a display device using characteristics of liquid crystal molecules which are aligned in different manners depending on a voltage applied. The LCD device can be driven with lower power than cathode-ray tube (CRT), and very advantageous for reducing size and thickness, thereby being concentrated as a flat panel display (FPD) device for computer monitors, TVs or the like, for example. In addition, the LCD device is light and thin so as to be easily portable, thereby being widely used for display devices of laptop computers, personal digital assistants (PDAs) and the like.
The LCD device is a device which includes two substrates each having an electrode and disposed to urge the two electrode face each other and a liquid crystal (LC) layer interposed between the two substrates, so as to control movement of liquid crystal molecules by use of an electric field generated by a difference of voltages applied to the two electrodes and adjust light transmittance accordingly, thereby realizing an image.
Meanwhile, the LCD device is generally a passive device which cannot emit light by itself, so it needs an independent light source. Therefore, in addition to a liquid crystal (LC) panel configured with the two substrates and the LC layer, a backlight for supplying light to a rear surface of the LC panel is disposed to urge light emitted from the backlight incident on the LC panel. Accordingly, the incident light can be adjusted according to the arrangement of liquid crystal, thereby displaying an image.
This type of LCD device is referred to as a transmission type LCD device. The transmission type LCD device uses an artificial light source, such as the backlight, so it can display a bright image even in a dark external environment. However, the backlight should be supplied with power. Accordingly, when the transmission type LCD device is used for a display device of a portable apparatus, it causes a relatively high power consumption.
Therefore, to make up for the drawbacks, a reflection type LCD device using an external light source without use of the backlight has been proposed. The reflection type LCD device operates using external natural light or artificial light so as to remarkably reduce the power consumption of the backlight. Consequently, it consumes relatively less power than the transmission type LCD device, which allows a long-time use, thereby being usually employed in portable devices, such as PDAs.
The reflection type LCD device does not include an independent light source, accordingly, it has an advantage in view of the low power consumption but also has a disadvantage in view of impossible use in a place without external light.
Hence, a transflective type LCD device which employs only the advantages of the reflection type LCD device and the transmission type LCD device has recently been introduced.
FIG. 1 is a planar view showing part of a display region of an LC panel in a transflective type LCD device according to the related art, and FIG. 2 is a sectional view taken along the line I-I of FIG. 1, which shows one pixel region.
As shown in FIG. 1, in each of N×N pixel regions vertically and horizontally arranged on the LCD device 1 is provided a thin film transistor (TFT) 10 located at an intersection between a gate line 3 for receiving a scan signal from an external driving circuit and a data line for receiving an image signal therefrom. The TFT 10 includes a gate electrode 11 connected to the gate line 3, a semiconductor layer 15 formed on the gate electrode 11 and activated when the scan signal is applied to the gate electrode 11, and a source electrode 12 and a drain electrode 13 both formed on the semiconductor layer 15. There is also provided in each pixel region a pixel electrode 18, which is connected to the source and drain electrodes 12 and 13 for receiving an image signal via the source and drain electrodes 12 and 13 in response to the activation of the semiconductor layer 15 so as to drive liquid crystal molecules (not shown).
The pixel region includes a transmission region T for transmitting light incident from a lower backlight (not shown), and a reflection region R for reflecting light incident from the exterior, and a reflecting layer 28 is provided in the reflection region R for reflecting light which is incident from the outside.
As shown in FIG. 2, the gate electrode 11 of the TFT and the gate line 3 are formed on the first substrate 20, and a gate insulating layer 22 is deposited on the entire first substrate 20. The semiconductor layer 15 is formed on the gate insulating layer 22 and the source and drain electrodes 12 and 13 are placed on the semiconductor layer 15, thereby configuring the TFT. Although not shown, the data line 5 is formed on the gate insulating layer 22.
Referring to FIG. 2, a passivation layer 24 is printed on the entire first substrate 20, and the pixel electrode 18 made of a transparent electrode, such as indium tim oxide (ITO), is formed on the passivation layer 24. As shown in the drawing, a contact hole 17 is formed at the passivation layer 24 for allowing an electrical connection between the drain electrode 15 of the TFT and the pixel electrode 18.
An insulating layer 26 is formed on the passivation layer 24 located on the transmission region T, and the reflecting layer 28 is formed on the insulating layer 26. Here, the insulating layer 26 has a contact hole 27 for allowing an electrical connection between the reflecting layer 28 and the pixel electrode 18.
The second substrate 40 includes a black matrix 42 and a color filter layer 46. The black matrix 42 is configured to prevent light leakage into a region (i.e., image non-display region) in which liquid crystal molecules are not driven. As shown, the black matrix 42 is mostly formed on the TFT region and between pixels (i.e., gate line and data line regions). The color filter layer 46 is configured with red (R), blue (B) and green (G) sub color filters so as to represent actual colors. The first substrate 20 and the second substrate 40 are bonded in an aligned state and a liquid crystal layer 50 is formed between the first and second substrates 20 and 40, thereby fabricating the LCD device 1.
Although not shown, a polarizer is disposed at an outer side of the LCD device 1 so as to polarize light incident on and output from the LCD device 1.
However, in the transflective LCD device with the structure, one pixel is divided into a reflection part and a transmission part, so, upon being driven in a reflection mode or a transmission mode, each pixel is substantially used by about 50%. Accordingly, an aperture ratio is drastically lowered upon operation in the reflection mode and the transmission mode, finally causing degradation of luminance, resulting in lowering an image display quality.